Some people argue that the Three Gorges Dam should have been constructed as an arch dam, applying the eggshell principle to withstand water pressure. In reality, the type of dam to be built depends on various factors. Arch dams transfer water pressure to both banks through their curved structure, making them suitable for narrow gorges and cost-effective. In contrast, gravity dams like the Three Gorges Dam resist water pressure by their own weight. They are massive in structure and require large quantities of construction materials. Gravity dams are structurally stronger than arch dams.
There is no need to worry excessively when the reservoir water level "exceeds the flood control level by X meters". During flood season, reservoirs discharge water in advance to the flood control level to reserve storage capacity for incoming flood peaks. For instance, the normal pool level of the Three Gorges Reservoir is 175 meters, and the flood control level is 145 meters. Therefore, "the water level of the Three Gorges Reservoir exceeds the flood control level by nearly 2 meters" means it reaches 147 meters, which is still far below the normal pool level.
A "100-year flood" is a statistical value, not an event that literally occurs once every 100 years. It is possible for two consecutive years to experience floods of 100-year magnitude.
Yang BiwangMaster of Physics, Brandenburg University of Technology (BTU), Germany
The flood control situation in the Yangtze River Basin has been relatively severe this year, with considerable losses in some areas. During this period, various rumors have spread online, some about the Three Gorges Dam and others about floods. Is an arch dam necessarily stronger than a gravity dam? Does a "100-year flood" really happen once a century? Let’s analyze them one by one.
1. Dam Type Depends on Multiple Factors; Arch Dams Are Not Stronger Than Gravity Dams
The Three Gorges Dam is a concrete gravity dam with a total axis length of 2,309.5 meters. It mainly consists of the flood discharge section, the powerhouse sections on both banks, and the non-overflow sections. Gravity dams rely on their own dead weight to hold back reservoir water, featuring a massive structure that consumes large amounts of building materials.
Some claim that "the Three Gorges Dam should be an arch dam using the eggshell principle to resist water pressure". It is easy to understand that arch structures can withstand pressure. However, choosing a dam type requires comprehensive consideration.
Arch dams transfer hydrostatic pressure to both banks through their curved structure. Thus, they are more suitable for narrow canyons, where steep rock walls on both sides can support the structure and stress. Arch dams are much thinner than other dam types, requiring far less construction materials and being more economical. For example, the Jinping I Hydropower Station Dam on the lower reaches of the Yalong River in Sichuan Province is an arch dam. It has a maximum central angle of approximately 93 degrees and a crest arch centerline length of about 552 meters, with a span of roughly 340 meters—tiny compared to the Three Gorges Dam’s 2,300-meter span.
Gravity dams are more robust than arch dams. It is far easier to destroy an arch dam than a solid gravity dam. Taking eggs as an analogy: although an egg can bear uniform gripping force, a concentrated impact on a single point will easily shatter the shell. Strategically, gravity dams are therefore safer.
2. No Need to Panic Over "Exceeding the Flood Control Level"
News reports often mention various water levels, such as the flood control level. Many people panic when seeing that the water level "exceeds the flood control limit". Here is a simple explanation of the different water levels of the Three Gorges Reservoir.
First, water levels in water conservancy and hydropower projects refer to altitudes, so "above sea level" is generally omitted. The Three Gorges Reservoir uses the Wusong Datum, with sea level at Shanghai’s Wusongkou as the zero point. A flood control level of 145 meters means the water surface is 145 meters above Wusongkou sea level, not a water depth of 145 meters.
The normal pool level of 175 meters is the highest water level for regular operation to meet benefits and disaster mitigation needs, serving as the upper limit for daily use; actual levels are usually lower.
The flood control level of 145 meters requires the reservoir to discharge water in advance during flood season, lowering the level to 145 meters to reserve capacity for upcoming flood peaks. When news reports state "the Three Gorges water level exceeds the flood control limit by nearly 2 meters", it means the level is about 147 meters—a normal level that causes no concern. Only if the level exceeds the normal pool level by nearly 2 meters would there be cause for alarm.
Some may ask: why not drain more water or even empty the reservoir? First, excessive drawdown would disrupt power plant operations and disrupt upstream navigation, risking vessel grounding. Second, inaccurate flood peak forecasts could leave the reservoir empty when no flood arrives, causing water shortages for downstream demand. Additionally, domestic water use for residents along the Yangtze and agricultural irrigation must be considered. The target water level is determined through comprehensive analysis, not arbitrary choice.
3. "100-Year Flood" Does Not Mean "Once in a Century"
Terms like "100-year flood" and "1,000-year flood" are probability-based. A 100-year flood refers to a flood of that magnitude or greater with a 1% annual probability, occurring once every 100 years on average—not exactly once per century.
For example, a hydrological station with 100 years of flood peak records can establish a relationship between discharge and frequency. Using formulas, the probability of a specific discharge can be calculated. Since expressing floods in cubic meters per second is unintuitive, return periods are used instead: 1% probability corresponds to a 100-year flood, and 0.1% to a 1,000-year flood.
Flood control requires coordination in both space and time. Some wonder why the Three Gorges discharges water when downstream pressure is high. The dam cannot fully close its gates; it must prepare for incoming flood peaks in the coming days. Furthermore, discharged water takes time to reach downstream cities. Flow velocity, discharge timing, and many other factors require complex calculation and comprehensive consideration.
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